The MRI apparatus is an apparatus that measures en NMR signal generated by the object, especially, the spin of nuclei which form human tissue, and images the shapes or functions of the head, abdomen, limbs, and the like in a two-dimensional manner or in a three-dimensional manner. In the imaging, different phase encoding is given to NMR signals by the gradient magnetic field and frequency encoding is also given to the NMR signals, and the NMR signals are measured as time-series data. The measured NMR signals are reconstructed as an image by a two-dimensional or three-dimensional Fourier transform.
When performing imaging on the basis of a predetermined pulse sequence in the above-described MRI apparatus, it is necessary to control the application time and strength of a gradient magnetic field accurately and freely, while selectively exciting a specific region. However, when a gradient magnetic field occurs, damping current is induced in a conductive structure around the gradient magnetic field coil. This is called an eddy current, and generates a magnetic field that changes spatially and temporally. As a result, the gradient magnetic field received by the object deviates from the ideal state, and this appears as various kinds of image quality degradation, such as image distortion, a reduction in the signal strength, and ghosting. Various methods for correcting the error magnetic field due to eddy current have been studied so far, and many have also been proposed as a patent.
On the other hand, when a gradient magnetic field is generated in the MRI apparatus, the Lorentz force due to the gradient magnetic field coil current spreads to the surrounding structure. The generation and dissipation of a gradient magnetic field in MRI imaging have a specific direction and occur frequently and periodically. Therefore, the force spreading to the surrounding structure also has directivity and periodicity. That is, it can be said that the MRI apparatus always “vibrates” in a certain direction and period while MRI imaging is being performed. In particular, this is a problem when the direction and period of vibration matches the natural frequency characteristics of the mechanical structure of the MRI apparatus. It is known that the vibrational error magnetic field caused by the resonance phenomenon of the mechanical structure has a strength having an influence on the image quality that cannot be neglected. MRI apparatuses in the related art have been designed firmly so that the resonance phenomenon does not occur or the mechanical structure is obtained in which the resonance phenomenon does not affect the image quality even if the resonance phenomenon occurs. In recent years, however, the need for a design allowing the occurrence of resonance has arisen from the perspective of product cost.
PTL 1 may be mentioned as one of the techniques for avoiding the vibrational error magnetic field due to the vibration of the mechanical structure. In the technique proposed in PTL 1, natural frequency information of a target MRI apparatus, which is obtained by performing measurement and analysis using a reference gradient magnetic field waveform, is stored in advance. In actual MRI imaging, the vibration of the mechanical structure caused by the execution of the pulse sequence is estimated on the basis of the measurement conditions set by the operator and the natural frequency information prepared in advance. Then, it is determined whether or not the estimated value of the vibration exceeds the allowable amount, and a change of the measurement conditions is prompted if the estimated value of the vibration exceeds the allowable amount.